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ANKARA, Turkey (CNN) -- The shootout outside the U.S. consulate in Istanbul which left six people dead was an "obvious act of terrorism," the U.S. ambassador to Turkey says.

Speaking to reporters in the Turkish capital of Ankara, Ross Wilson said he had asked Turkey to implement additional security measures after gunmen Wednesday pulled up in a car and opened fire at a police security checkpoint at the consulate entrance.

"I'm not in a position to speculate on who this is or why they have carried out this action," Wilson said. "But any time there is an attack on diplomatic establishment... (it) is more or less by definition is an act of terrorism."

"Our countries will stand together to confront this as we have confronted some other problems in the past," he added. Watch emergency staff helping victim »

Three police officers and three assailants were killed in the shootout near the U.S. consulate in Istanbul, the city's governor said.

Two other police officers were wounded in the attack. A U.S. consulate official said no American citizens or employees were hurt. Are you there? Send photos, videos

Gunmen pulled up in a white car and opened fire at a police security checkpoint at the outer entrance of the consulate, Istanbul Gov. Muammer Guler told reporters at the scene.

Police fired back, resulting in a three- to five-minute gun battle, Ivan Watson, a journalist with National Public Radio reporting from the scene, told CNN.

Guler said the dead included three police officers and three assailants. Authorities did not immediately know whether the attackers were affiliated with any organization, he said.

People waiting to obtain visas inside the heavily fortified building were not hurt. The outer entrance is more than 30m (100ft) from the main building which sits atop a hilltop.

At least three bodies remained on the ground as ambulances pulled up and police cordoned off the area with yellow tape and waved off onlookers.

The most recent attack on a foreign mission in Turkey was in November 2003 when a string of bombings in Istanbul targeted the British consulate, along with two synagogues and a British-owned bank. The blasts killed more than 70 people, including the British consul general, and wounded hundreds.

Turkey is a secular country that is predominantly Muslim. There has been a lot of tension in the country between secularist and traditional Muslims, and the state has been battling Kurdish separatists for many years.